Users are having a hard time confirming their subscription
Postmatic API keys are domain specific. This means that if you register for a key for mygreatsite.com the key will only be good for that domain.
In the event that you change your site url your outgoing mail may still send, however incoming replies (such as confirmations and other commands) will not work. This can lead to a confusing experience of users being invited to confirm their subscription but doing so does not work.
This problem commonly occurs when registering for an API key for a development or staging site.
How to update your site url
If your site url changes please contact support to let us know your old url and what you would like it updated to.